JULY 1999 NOTES AND CORRESPONDENCE 1693

Observations of Nontornadic Low-Level and Attendant Tornadogenesis Failure during *

R. J. TRAPP NOAA/National Severe Storms Laboratory, and Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma

19 February 1998 and 13 July 1998

ABSTRACT Three storms intercepted during the Veri®cation of the Origins of Rotation in Tornadoes Experiment generated a moderate-to-strong within the lowest several hundred meters above the ground and qualitatively appeared capable of tornadogenesis, yet did not produce a . Such novel observations of what is considered ``tornadogenesis failure'' are documented and used to show the insuf®ciency of a low-level mesocyclone for tornadogenesis. Possible modes of failure are discussed.

1. Introduction and in other ways appeared ``primed for tornadogene- sis'' (Brandes 1993), yet did not produce tornadoes. In The Veri®cation of the Origins of Rotation in Tor- other words, existence of a low-level mesocyclone was nadoes Experiment (VORTEX; Rasmussen et al. 1994) an insuf®cient condition for tornadogenesis. One may was conducted during the spring of 1994 and 1995 in argue, then, that theories of low-level mesocyclogenesis the southern Great Plains of the United States. The ob- (e.g., Rotunno and Klemp 1985; Davies-Jones and jectives of this validation experiment were driven by a Brooks 1993; Brooks et al. 1993, 1994) fall short of set of hypotheses (see Rasmussen 1995) that concerned explaining the details of tornadogenesis, as underscored (i) the initiation of tornadic storms; (ii) low-level me- by the idealized modeling results of Trapp and Fiedler socyclogenesis, tornadogenesis, and the role(s) of me- (1995). Hence, another layer of complexity must be ad- soscale and stormscale boundaries in each; and (iii) the dressed by theories of tornadogenesis. dynamics of tornadoes and their associated boundary The aforementioned nontornadic storms are consid- layers. ered examples of ``tornadogenesis failure,'' an arguably VORTEX data presented herein provide additional subjective classi®cation introduced here to provide a observational basis for the sentiment expressed in Da- basis for comparison with tornadic storms and to help vies-Jones and Brooks (1993): ``in perhaps the least clarify tornadogenesis mechanisms. This concept of known process, a tornado develops within the meso- ``failure'' is attributed to Brooks et al. (1993), albeit in cyclone.'' Indeed, several storms intercepted during the context of low-level mesocyclogenesis. These au- VORTEX generated moderate to strong low-level me- thors identi®ed failure modes in which a strong (vertical 1 socyclones that were sustained for a Ն15 min period, , ␨ ϳ 0.01 sϪ1) midlevel and/or low-level me- socyclone neither developed nor persisted in a numer- ically simulated storm; a failure mode was presumed to preclude the formation of a long-lived, signi®cant tor- 1 This approximates the minimum amount of time required to am- 2 plify, through vortex stretching with a constant divergence of Ϫ5 ϫ nado. A proper balance between two opposing factors 10Ϫ3 sϪ1, vorticity of a strong mesocyclone (␨ ϳ 1 ϫ 10Ϫ2 sϪ1)to that govern rear-¯ank gust-front movement, namely the that of a tornado (␨ ϳ 1sϪ1); these vorticity and divergence values environmental in¯ow and the cold-air out¯ow, was are well within the range of observed values (e.g., Lemon and Doswell found to be of critical importance in low-level meso- 1979). and sustenance [and in the sustenance of the parent storm itself; Weisman and Klemp (1982)]. * A portion of this research was presented at the 28th Conference The VORTEX observations imply a new challenge on Radar Meteorology, Austin, Texas.

Corresponding author address: Dr. R. Jeffrey Trapp, NSSL, 1313 Halley Circle, Norman, OK 73069. 2 The gridpoint spacing was too coarse to resolve tornado-scale E-mail: [email protected] motions.

᭧ 1999 American Meteorological Society

Unauthenticated | Downloaded 09/25/21 10:12 PM UTC 1694 MONTHLY WEATHER REVIEW VOLUME 127 to operational forecasters in their efforts to discriminate, TABLE 1. Approximate distance (km) of each storm's mesocyclone for warning purposes, storms that are likely to produce from the airborne Doppler radar and nearest WSR-88D, at analysis time T. Vertical resolution is estimated for the airborne Doppler radar tornadoes from those that will not. A means to discrim- at the indicated range, assuming an azimuthal sampling interval or inate unambiguously between tornadic and nontornadic sweep-angle resolution of 1.2Њ. Asterisk denotes tornadic case. storms has yet to be demonstrated in the literature, hence Distance to Vertical motivating related research at the National Severe airborne data Distance to Storms Laboratory (NSSL) involving VORTEX data radar spacing Nearest WSR-88D and complementary numerical modeling. This report Case (km) (km) WSR-88D (km) provides the underpinning of some of that work by es- 0429 14 0.29 KFWS 135 tablishing existence of nontornadic low-level mesocy- 0512 10 0.21 KDDC 160 clones and attendant tornadogenesis failure during 0522 16 0.34 KAMA 125 VORTEX. 0529* 9 0.19 KFWS 155 0417* 10 0.21 KTLX 140 A description of the datasets and analysis method 0516* 19 0.40 KDDC 80 used toward this end is provided in section 2. In section 3, evidence to support the classi®cation of failure within three nontornadic storms is presented. A diagnostic comparison with three tornadic storms is used in section observations) and after which low-level vertical vortic- 4 to explore and provide additional clues on possible ity diminishes, is designated as time of tornadogenesis modes of failure. Some preliminary remarks on failure failure. A tornado is de®ned as in the Glossary of Me- modes are discussed in section 5. teorology. Although each instance of tornadogenesis failure is by de®nition a nontornadic event, it is not assumed herein that all nontornadic storms are instances 2. Data description of tornadogenesis failure; ``nontornadic'' carries with it Airborne Doppler radar (ADR) observations, ob- no distinction about the existence of low-level rotation, tained by a helically scanning X-band radar mounted in which, on some scale larger than that of the tornado, is the tail of one of the NOAA P-3 aircrafts, make up the necessary for tornadogenesis. primary data source. When employing either the fore± WSR-88D data are used to identify times of maxi- aft scanning technique or the all±fore, all±aft scanning mum low-level rotational velocity and thus guide which technique, the P-3 is capable of gathering pseudo-dual- ADR ¯ight legs to analyze. Using calculations of ver- Doppler data during a ¯ight leg (Jorgensen et al. 1996). tical vorticity from the pseudo-dual-Doppler synthe- A typical leg lasts ϳ5 min and is ¯own at a distance sized winds, tornadogenesis failure time is then veri®ed, of ϳ10±20 km from the storm and at an altitude of ϳ1 or adjusted if necessary. Additional analyses are pro- km above ground level. The P-3 aircrafts also are duced at ϳ5 and ϳ10 min prior to failure time (here- equipped with a lower-fuselage (LF)-mounted C-band inafter denoted as T5 and T10). conventional radar. This radar scans at a rate of two revolutions per min and collects re¯ectivity information 3. Observational evidence of tornadogenesis failure at low elevation angles with respect to the quasi-hori- zontal plane containing the aircraft ¯ight track. The It is prudent to recall ®rst the pioneering efforts of Weather Surveillance Radar-1988 Doppler (WSR-88D) Stout and Huff (1953), Browning and Donaldson nearest to each storm supplements the LF and airborne (1963), Chisholm and Renick (1972), Marwitz (1972), Doppler radars. For reference, approximate distances of and Lemon (1980). These researchers, among others these radars from the mesocyclone center of each storm, [see the reviews by Donaldson (1990) and Burgess and at the time of tornado formation or failure, are listed in Lemon (1990)], introduced storm air¯ow mod- Table 1. Details of the ADR data analysis are provided els and radar-observable characteristics such as pendant in the appendix. and hook echoes from which rotation may be inferred; The criterion for VORTEX case consideration and bounded weak echo regions (BWER) as a means to selection is existence of pseudo-dual-Doppler data on identify regions of strong updrafts; rotation (low-alti- the storm, at least 10±15 min prior to the time of its tude convergence, storm-top divergence) signatures in failure. To provide a basis for comparison with tornadic mean Doppler velocity that depict mesocyclones (up- storms, failure is de®ned to be the lack of tornado for- drafts); and spatial correlations of a mesocyclone sig- mation within a strong (␨ Ն 0.01 sϪ1) low-level me- nature and BWER that indicate a rotating updraft. socyclone whose life cycle is Ն15 min, and furthermore Such characteristics have been used in descriptions within a storm possessing other characteristics shown of supercell storm life cycles. The stage that heralds to be associated with a transition to a tornadic phase tornado formation is of relevance in the present dis- (Klemp 1987; see section 3). The time of occurrence of cussion. Brandes (1993), for example, described a ``ma- peak low-level vertical vorticity, which in these cases ture stage'' as ``that critical period in storm evolution corresponds to the time at which tornadogenesis appears at which the basic updraft and vertical vorticity patterns imminent (from subjective visual and/or weather radar associated with have evolved and the storm

Unauthenticated | Downloaded 09/25/21 10:12 PM UTC JULY 1999 NOTES AND CORRESPONDENCE 1695 is primed for tornadogenesis.'' A rudimentary hook kinematic occlusion of the mesocyclone at time T. Qua- echo, WER or BWER, rainy rear-¯ank downdraft and si-vertical tail radar scans of the three storms depict the associated gust front, and an arc-shaped updraft that presence of a deep, echo-free vault, or BWER, and correlates spatially with vertical vorticity, are qualitative therefore extensive updraft (Fig. 2a). Signi®cant hail features that tend to be observed during and near the (diameter Ͼ 4.5 cm) was reported in each case. WSR- end of the mature stage. The formation of a tornadic 88D data (see Table 1) con®rm a spatial correlation of vortex signature (TVS; Brown et al. 1978) aloft and WER or BWER and mesocyclone, at some time during within the mesocyclone also tends to precede torna- or preceding failure. Also, data collected from the Am- dogenesis [though not necessarily in all cases; see Trapp arillo, Texas, WSR-88D (KAMA), for example, show and Davies-Jones (1997)]. In terms of kinematic quan- the presence of a TVS at ϳ2 km above the ground within tities, the mature stage of the 8 June 1974 Harrah, the Shamrock mesocyclone (Fig. 2b). Lastly, observa- Oklahoma, tornadic storm was characterized by ␨ ϳ 1.5 tions of hook echoes from the LF radar at low altitude ϫ 10Ϫ2 sϪ1 and mean divergence of ϳϪ50 ϫ 10Ϫ4 sϪ1, support existence of low-level mesocyclones (Fig. 2c). at an altitude of 300 m (cf. Brandes 1993, his Figs. 6 Within the context of the perceived understanding of and 7). tornadogenesis, these data provide ample evidence to Other descriptions can be found as well. Lemon and support the characterization of tornadogenesis failure. Doswell (1979) detailed a ``collapse phase'' during LF radar data provide some clues about the nature of which time ``the strongest tornadoes and straight-line each storm's evolution and failure mode. In particular, winds occur.'' This is characterized by a dissipating scans of the Sherman storm (Fig. 2c) portray well this BWER and the development of a divided mesocyclone storm's cyclic mesocyclogenesis, and exemplify the dis- structure, which are accompanied by the descent of the cussions of Burgess et al. (1982) and more recently, mesocyclone and rear-¯ank downdraft to the surface, Adlerman et al. (1996). This cyclic behavior is consis- and also the development of a TVS aloft. Klemp (1987) tent with the evolution of the ``low-shear,'' out¯ow- drew upon Lemon and Doswell's discussion as well as dominated case of Brooks et al. (1994), and also with numerical storm simulation results to describe a tran- the relatively high values of radar re¯ectivity within the sition of a storm to its tornadic phase. This includes a Sherman mesocyclone core. rapid increase in low-level rotation. An adverse vertical pressure gradient owing to the relatively stronger low- level mesocyclone forces a downdraft that in turn leads 4. Diagnostic comparison to the onset of mesocyclone occlusion (see also Klemp Three tornadic storms intercepted during VORTEX and Rotunno 1983). now are introduced for comparative purposes: the 29 With these descriptions in mind, three VORTEX cas- May 1994 tornadic storm near Newcastle, Texas; the 17 es3 are considered herein as illustrations of tornadoge- April 1995 tornadic storm near Temple, Oklahoma; and nesis failure: the 29 April 1995 nontornadic storm near the 16 May 1995 tornadic storm near Garden City, Kan- Sherman, Texas; the 12 May 1995 nontornadic storm sas. An intensity estimate (Fujita 1981) of F3 was as- near Hays, Kansas; and the 22 May 1995 nontornadic signed to the Newcastle tornado, F1 to the Temple tor- storm near Shamrock, Texas. To reiterate, a nontornadic nado, and F2 to the Garden City tornado. storm that appears to evolve to a state ostensibly Prima facie, the tornadic and nontornadic storms ``primed for tornadogenesis,'' and in particular, that pos- share many of the same traits (Fig. 3). To help identify sesses a moderate-to-strong mesocyclone in the lowest differences, several kinematic quantities5 are computed several hundred meters above ground, characterizes tor- using the radar re¯ectivity and three-dimensional wind nadogenesis failure. analyses synthesized from the ADR. These are diag- Horizontal sections of re¯ectivity factor and storm- nostic, so as to show trends and ``effect'' rather than relative winds, at z ϭ 1 km and analysis times T5 and ``cause,'' yet hopefully provide further clues regarding T, are presented in Fig. 1.4 Each storm exhibits, to vary- the modes of failure. Computations using analyses that ing degrees of intensity, a low-level mesocyclone and are plagued with missing data in crucial areas (e.g., in a rainy downdraft to its rear. An associated rear-¯ank and around the mesocyclone) or that have the potential gust front arcs outward from the mesocyclone center; in no storm has the gust front advanced to promote a

5 An attempt was made to assess the magnitude and orientation of near-ground, horizontal buoyancy gradients (and therefore estimate 3 The 8 June 1995 nontornadic storm near Elmwood, Oklahoma, baroclinically generated horizontal vorticity) with measurements a VORTEX case currently under investigation by D. O. Blanchard made by a ``mobile mesonet'' [15±20 vehicles instrumented with of NSSL, also exempli®es tornadogenesis failure (D. O. Blanchard temperature, pressure, humidity, and wind sensors at ϳ3 m; see Straka 1998, personal communication). et al. (1996)]. Unfortunately, these gradients, as anticipated by the 4 Accounting for the approximate distance of each storm's meso- modeling and theoretical studies of Rotunno and Klemp (1985) and cyclone from the airborne Doppler radar (see Table 1), 0.5 km is a Davies-Jones and Brooks (1993), were inadequately sampled by the conservative estimate of the height of the ®rst data point above the mobile mesonet (due to poor road networks, etc.) in the cases dis- ground level. cussed herein, at and prior to time T.

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FIG. 1. Horizontal sections of storm-relative horizontal velocity and radar re¯ectivity factor (in dBZ), at 1.0- km altitude, for the nontornadic storms at analysis times T5 and T. Radar re¯ectivity factor is contoured at 10-dBZ increments, and vectors are plotted at every other grid point. (a) The 29 Apr 1995 Sherman, Texas, nontornadic storm at 0023:00 (T5) and 0028:45 (T). (b) The 12 May 1995 Hays, Kansas, nontornadic storm at 2252:00 (T5) and 2256:20 (T). (c) The 22 May 1995 Shamrock, Texas, nontornadic storm at 2349:30 (T5) and 2358:00 (T). Times are in UTC unless otherwise indicated.

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FIG. 2. Scans of (a) radar re¯ectivity factor (dBZ) of the 29 Apr 1995 Sherman, Texas, nontornadic storm at 0028:15, from the tail radar; (b) re¯ectivity and radial velocity (m sϪ1) of the 22 May 1995 Shamrock, Texas, nontornadic storm at 2359:18, from WSR-88D KAMA at 0.5Њ elevation; and (c) re¯ectivity of the 29 Apr 1995 Sherman, Texas, nontornadic storm at 0027:19, 0028:54, 0041:02, and 0046:35, from the lower-fuselage radar. for relatively greater vertical velocity uncertainties due tornadic mesocyclones have greater vorticity at time T to large distances between radar and storm are omitted. than do the nontornadic mesocyclones.

Accounting for errors related to the radar system hard- Next, consider estimates of the mean core radius, r c, ware, ¯ow evolution and improperly corrected storm within each low-level mesocyclone. Upon de®ning a motion during pseudo-dual-Doppler data collection, im- circulation center (x 0, y 0) using ␨max and |VH|min, a tan- properly dealiased velocity data, and synthesis of Car- gential velocity can be computed: tesian velocity components from pseudo-dual-Doppler V ϭϪu sin␪ ϩ ␷ cos␪, (1) radars separated by 40Њ in azimuth (see Jorgensen et al. ␪ Ϫ1 1996), the values presented below are estimated to have where ␪ ϭ tan [(y Ϫ y0)/(x Ϫ x0)] is the azimuth angle, errors of approximately Ϯ10%. and u and ␷ are the (synthesized) Cartesian velocity

It is logical to begin with an evaluation of maximum components. Mean core radius r c is determined by ®nd- low-level (0 Ͻ z Յ 1 km) vertical vorticity, ␨max, which ing the radius of maximum tangential velocity about is graphed for each case at each of the three analysis (x 0, y 0) at each azimuth angle ␪, and then averaging times (Fig. 4). Although the general trend is an increase over ␪. in ␨max prior both to tornado formation and failure, the In Fig. 5a, r c is plotted as a function of ␨max at analysis

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FIG. 3. As in Fig. 1 except for three tornadic storms. (a) The 29 May 1994 Newcastle, Texas, tornadic storm at 2301:15 (T5) and 2304:45 (T). (b) The 17 Apr 1995 Temple, Oklahoma, tornadic storm at 2251:52 (T5) and 2258:50 (T). (c) The 16 May 1995 Garden City, Kansas, tornadic storm at 2318:43 (T5) and 2324:00 (T).

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FIG. 4. Maximum, low-level (0 Ͻ z Յ 1 km) vertical vorticity,

␨max, for each nontornadic (boldface) and tornadic (italicized) case, at the three analysis times (T10, T5, and T; see text). time T. Clearly, the tornadic mesocyclones have smaller core radii and greater low-level vertical vorticity than do the nontornadic mesocyclones. This statement also holds true at analysis time T5 and is evident qualitatively in the storm-relative winds of the Garden City (tornadic) and Shamrock (nontornadic) storms analyzed at both times (Figs. 1c and 3c). An intuitive and theoretically consistent result is shown when r c is plotted as a function of the maximum, mean, low-level vertical vortex stretching, (Fig. Ϫ␨␦ FIG. 5. Mean, low-level mesocyclone core radius r c at analysis

5b). Here, ␦ is horizontal divergence, and the mean is time T, as a function of (a) ␨max for each case, and of (b) maximum, constructed from all values within the nominal threshold mean vertical vortex stretching, Ϫ␨␦ for two tornadic (0529, 0516) of ␨ Ͼ 1 ϫ 10Ϫ2 sϪ1. The four cases suitable for this and two nontornadic cases (0429, 0512). Tornadic cases are indicated by ࠘. computation exhibit, at T, a well-behaved progression from large core radii and weaker vortex stretching in the nontornadic mesocyclones to smaller core radii and stronger stretching in the tornadic mesocyclones. Said mesocyclone, and thus a cessation or disruption of vor- another way, parcels that nearly conserve angular mo- tex stretching. mentum penetrate closer to the central axis of the tor- An alternative means of diagnosing the genesis or nadic mesocyclones, resulting in large tangential veloc- failure in the six cases is provided through calculations ities. of swirl ratio S (Davies-Jones 1973). This approach fol- Correlations (or lack thereof) of vertical velocity and lows Brandes (1978) and Barnes (1978), who used swirl vertical vorticity corroborate the results in Fig. 5. Fol- ratios computed with observed winds (by dual-Doppler lowing Droegemeier et al. (1993), a correlation coef- radar and instrumented mesonet towers, respectively) to ®cient ␳(w, ␨) is de®ned at each level 0 Ͻ z Յ 1km explain tornado development and subsequent behavior. as After Davies-Jones (1973), swirl ratio can be written as ͗w␨͘ tan␣ ␳(w, ␨) ϭ , (2) S ϭ , (3) ͗w͗͘␨͘ 2a where ͗͘represents a horizontal average across a me- where a ϭ h/r 0 is the ratio of storm in¯ow depth (h) Ϫ2 Ϫ1 socyclone (de®ned by ␨ Ͼ 1 ϫ 10 s ). Interestingly, to nominal updraft radius (r 0). In¯ow depth is approx- the maximum low-level correlation decreases with time imated as in Barnes (1978) using the level of free con- by more than 50% prior to tornado failure, yet changes vection (LFC) from a rawinsonde observation (raob) in very little prior to tornadogenesis (Fig. 6). This is in- the environment of each storm. As demonstrated in Fig. dicative of a decoupling between low-level updraft and 7, r0 is estimated using contoured ®elds of azimuthally

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stagnant core, p 0 is the constant pressure in the irrota- tional region outside the core, and ␳ is the density (Lew- ellen 1976). For a ®xed core pressure de®cit, the core radius adjusts to its minimum value so as to maximize the volume ¯ow rate, yet increases with increasing swirl ratio. The stagnant vortex model does not apply at low swirl when vortices can be laminar upstream of an axial stag- nation point and turbulent core (Davies-Jones 1973). However, the model and chamber experiments do well at explaining the fully turbulent vortices. In the Ward tornado chamber for example, a single, wide, fully tur- bulent vortex can exist in a steady state, for certain moderate values of S less than a critical swirl ratio (S*, which is dependent on the Reynolds number, R) (see Fig. 4d in Ward 1972). Snow (1982) describes this as a vortex at ``intermediate swirl,'' the core has broad- ened, yet (weak) rising motion still exists along the FIG. 6. Maximum, low-level correlation coef®cient ␳(w, ␨) between central axis (see his Fig. 8). To the extent that such a vertical velocity w and vertical vorticity ␨, at analysis times T5 and T, for two nontornadic (0429, 0522; boldface) and two tornadic (0529, vortex is analogous to the three nontornadic, broadly 0516; italicized) cases. rotating mesocyclones, one may state that the swirl in these cases destines the mesocyclones to resist tornado- scale vortex formation. This statement comes with two caveats. First, unlike averaged vertical velocity w and radial velocity V r, where in the chamber and numerical models thereof, the swirl ratio associated with the parent is unsteady Vr ϭ u cos␪ ϩ ␷ sin␪. and therefore is a manifestation of storm-scale processes In¯ow angle at a given time. Second, the chamber/numerical models inadequately treat (and ADR observations fail to re- Ϫ1 ␣ ϭ tan (V ␪/V r) solve) Reynolds number±dependent boundary layer pro- is determined as in Brandes (1978) using azimuthally cesses that play an important role in tornado dynamics averaged tangential velocity [Eq. (1)] and radial veloc- (see, e.g., Snow 1982). This was stressed by one of the reviewers, who noted furthermore that if these processes ity, evaluated at r0 and h/2. Information regarding the raob for each case can be found in Table 2. Values of are simulated suf®ciently well, the models may reveal certain atmospheric values of R that allow strong radial h, r0, ␣, and S at analysis time T are listed in Table 3. At time of tornado formation or failure, the three in¯ow to develop in the surface layer and subsequently nontornadic storms have large swirl ratios and, with converge the otherwise nontornadic low-level meso- exception of the Garden City storm, the tornadic storms cyclone of an intermediate swirl vortex into a tornado. have smaller swirl ratios (Fig. 8). Results from labo- In this regard, tornadogenesis failure is dependent on R ratory model simulations as well as theory are invoked as well as S. Ultimately, the discussion of failure must to show the consistency of large (smaller) swirl ratio relate atmospheric conditions to particular values of S with large (smaller) core radii found in the nontornadic and perhaps R. (Newcastle and Temple tornadic) mesocyclones (see The Garden City storm and its thermodynamic en- Figs. 5 and 8, and Table 3). vironment foster the formation of a two-celled meso- Turbulent vortices simulated in tornado chambers ex- cyclonic vortex (i.e., down¯ow along the central axis, hibit a functional dependence of core radius on swirl terminating in low-level radial out¯ow that turns vertical ratio (Davies-Jones 1973; Jischke and Parang 1974; Ro- in an annular updraft at an outer radius) that apparently tunno 1977). An inviscid, potential ¯ow model of a exceeds some critical swirl ratio. As in laboratory vor- stagnant core vortex aids the interpretation of the ex- tices at S Ն S* and in multiple vortex tornadoes (Davies- perimental data. Upon using Bernoulli's theorem and Jones 1973; Jischke and Parang 1974; Church et al. applying a variational principle, it can be shown that 1979), a downdraft, presumably owing to a downward- directed vertical pressure gradient force, supplants up- 1/2 ward motion along the Garden City mesocyclone's cen- rcm 2SQ ␳ ϭ , (4) tral axis at times T10 (not shown) and T5 (Figs. 1c and rr2 2(p Ϫ p ) 000[]c 9). According to Rotunno (1984), the two-celled vortex where rcm is the minimum core radius, r 0 is the radius is unstable to three-dimensional perturbations, and the of the updraft hole [as in Eq. (3)], Q is the updraft instability is manifest greatest in azimuthal wavenumber volume ¯ow rate, pc is the constant pressure inside the two. These subsidiary vortices form in the annular

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FIG. 7. Azimuthally averaged (about the mesocyclone center) values of (a) tangential velocity, (b) radial velocity, and (c) ver- tical velocity, at analysis time T for the 12 May 1995 Hays, Kansas, nontornadic storm. Contour interval is 1 m sϪ1 in (a) and (c), and 0.5 m sϪ1 in (b). Extrapolation of contours to the ground level is for presentation only.

TABLE 2. A few relevant details concerning the raob chosen to TABLE 3. Values of storm in¯ow depth (h), updraft radius (r0), represent the environment of each storm. The separation distances azimuthally averaged tangential velocity (V r) evaluated at r0 and h/ Ϫ1 and times are determined with respect to the mesocyclone center at 2, and in¯ow angle ␣ ϭ tan (V ␪/V r), used to determine swirl ratio analysis time T. Asterisk denotes tornadic case. (S) for each case at analysis time T. Mean core radius in the ratio

r c/r0 is as presented in Fig. 5. Asterisk denotes tornadic case. Raob±storm

separation time (h), CAPE LFC V ␪ V r Ϫ1 Case distance (km) (J kg ) (km) Case r0 (km) h (km) (r0, h/2) (r0, h/2) Src/r0 0429 2.5, 45.0 2630 1.5 0429 3.0 1.5 14.0 Ϫ1.5 9.0 0.87 0512 1.5, 80.3 1090 1.6 0512 2.5 1.6 14.0 Ϫ2.0 5.5 1.4 0522 3.0, 54.0 2300 2.3 0522 3.5 2.3 18.0 Ϫ3.5 3.9 1.16 0529* 0.25,130. 3380 2.0 0529* 3.0 2.0 10.0 Ϫ3.75 2.0 0.70 0417* 0.33, 45.7 1810 2.3 0417* 4.0 2.3 6.0 Ϫ15.5 0.3 0.65 0516* 0.3, 70.9 2470 2.3 0516* 3.0 2.3 17.0 Ϫ1.5 7.4 0.96

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other uncertainty to the decision process, and complicate the tornadic/nontornadic storm discrimination problem. It is possible to speculate on a number of tornado- genesis failure modes that can be addressed in future studies. Certainly, a storm that becomes ``out¯ow-dom- inated'' exempli®es one mode. As discussed in Brooks et al. (1993, 1994), the processes that give rise to low- level mesocyclogenesis also disrupt the subsequent in- tensi®cation of associated low-level vorticity into a tor- nado-scale vortex. This means of failure presumably is quite time sensitive. Noting the analyses presented in sections 3 and 4, it is clear that not every nontornadic storm ``fails'' via mesocyclone undercut by cold-air out- ¯ow. Recall the distinct modes of tornado formation within a single-celled mesocyclone, as described by Trapp and Davies-Jones (1997): (i) mode I, in which the embryonic tornado develops aloft within the parent mesocyclone, then gradually descends to the ground via a process FIG. 8. Swirl ratio S (see text), evaluated at analysis time T, for known as the ``dynamic pipe effect'' (Leslie 1971); (ii) each nontornadic (boldface) and tornadic (italicized) case. mode II, in which the embryonic tornado forms uni- formly throughout a several kilometer depth or very near the ground, then contracts rapidly into a tornado. updraft ring that surrounds the central downdraft (see Both have corresponding failure modes. Mode I should Rotunno 1984, his Figs. 9 and 11). fail if the vertical stability in the convective boundary Brandes (1978) and later Rotunno (1984) hypothe- layer is too large. The result is an embryonic tornado sized that a subsidiary vortex in the two-celled meso- suspended above the ground. Mode II should fail if the cyclone of the 8 June 1974 Harrah, Oklahoma, storm primary meridional (radial±vertical) circulation asso- contracted into the Harrah tornado. Wakimoto and Liu ciated with the updraft is too weak, particularly at low (1998) applied this theory to their analysis of the Na- altitudes, to contract ambient rotation (the mesocyclone) tional Center for Atmospheric Research (NCAR) Electra into a tornado within tens of minutes. This also may be Doppler radar data collected on the Garden City storm interpreted as a cause for the intermediate swirl vortex, and tornado. Although the spatial and temporal data which resists tornado-scale vortex formation (for certain sampling of the NOAA P-3 data shown here are too values of Reynolds number, as discussed previously). large to afford a con®rmation of Wakimoto and Liu's The primary meridional circulation is driven in part by conclusion, it is nevertheless supported by visual ob- buoyancy forces, the vertical integral of which corre- servations by VORTEX personnel of tornadogenesis sponds to the convective available potential energy near the periphery of the mesocyclone rather than in its (CAPE), and also by upward-directed vertical pressure center. gradient forces (VPGFs) owing to interactions between the updraft and ambient . Following recent 5. Remarks results of McCaul and Weisman (1996), it is suspected that these favorable VPGFs and also the height of max- Six storms intercepted during VORTEX were con- imum positive buoyancy are more relevant to failure sidered. Each of the three tornadic and three nontornadic mode II than the value of CAPE alone. storms generated and sustained a mesocyclone in the The frequency with which tornadoes develop within lowest several hundred meters above the ground. Sig- two-celled mesocyclones is unclear, as is the frequency ni®cantly, this statement proves that mere existence of of two-celled mesocyclone formation itself. It is reit- a low-level mesocyclone was insuf®cient for tornado erated that the critical swirl ratio S*, the theoretical formation in the nontornadic cases. value at which two-celled vortices develop, is a function The percentage of low-level mesocyclones associated of Reynolds number and should vary from storm to with tornadoes has yet to be presented in the formal storm. Inability of the storm to attain S* represents in literature; it is presumed to be greater than 30%±50% some sense yet another mode of failure. Meteorological [the approximate number of tornadic midlevel meso- conditions in the near-storm, mesoscale environment cyclones; Burgess and Lemon (1990)], but less than (which perhaps include the effects of neighboring or 100%. Assuming that detection of a low-level meso- preexisting storms) modulate this and the other failure cyclone (when possible) weighs heavily in Doppler-ra- modes in ways yet to be determined. dar-based tornado warning decisions, these observations Much of the data collected during VORTEX lack the of nontornadic low-level mesocyclones introduce an- spatial and temporal resolution needed for a rigorous

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FIG. 9. As in Fig. 7 except at analysis time T5, and for the 16 May 1995 Garden City, Kansas, tornadic storm.

investigation of these and other conjectured failure Acknowledgments. The author is grateful to H. modes. Thus it is premature at this time to attempt to Brooks, C. Doswell, E. Rasmussen, T. Shepherd, I. Wat- assign a particular mode to a given storm. The inves- son, and C. Ziegler (NSSL), for assistance and enlight- tigative problem is, however, amenable in many ways ening discussions on this research. The author also ben- to experiments with a numerical cloud model since tor- e®ted from comments by the two reviewers and con- nadogenesis failure ultimately is related to meteorolog- versations with D. Dowell, S. Weygandt, and J. Wurman ical conditions in the near-storm, mesoscale environ- (University of Oklahoma). R. Davies-Jones (NSSL) is ment, as just mentioned. This likely will be the future recognized for suggesting and helping interpret the swirl direction of the research theme introduced here, in what ratio calculations and also serving as the author's mentor can be considered a logical extension of the work of while a National Research Council±NOAA Postdoctoral Brooks et al. (1994). Research Associate. Funding and support for VORTEX

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®eld operations were provided by NOAA/NSSL, Center Chen (1982), except here a mean motion derived from for the Analysis and Prediction of Storms at the Uni- the mesocyclone track over a 30-min period is used. versity of Oklahoma (NSF ATM 912-0009), and the Errors in the wind synthesis that are associated with Graduate College at the University of Oklahoma. storm evolution are not correctable. These arise because of the time lag between the fore and aft beam pair com- prising a pseudo-dual-Doppler observation at a point. APPENDIX Cartesian velocity components (u, ␷) are synthesized Airborne Doppler Radar Analysis Method from the radial velocities of the fore and aft scans in a manner similar to that outlined in numerous articles The airborne Doppler radar data analysis begins with (e.g., Brandes 1977; Ray et al. 1980). Vertical velocity a mapping from an aircraft-relative coordinate system (w) is determined from the anelastic continuity equation, to an earth-relative coordinate system, as well as a cor- using an explicit technique discussed by Gal-Chen rection for aircraft motion applied to the radial velocity. (1983, unpublished manuscript), Sperow (1995), and These are derived by Lee et al. (1994) and performed Sperow et al. (1995). The integration is taken from the using software developed by Oye et al. (1995). Because ground (where w is assumed to be zero) upward; errors of infrequent updates of latitude and longitude by the in w accumulate with increasing height, but do not reach onboard computer, it is also necessary during postpro- severe levels because of the relatively shallow domain cessing to correct the latitude and longitude stored in height of 3 km. The interested reader is referred to Ray each radar beam data header (E. Rasmussen 1996, per- et al. (1980) for estimates of vertical velocity error var- sonal communication). Re¯ectivity and velocity data are iances due to upward integration of the continuity equa- then edited manually with NCAR's Research Data Sup- tion. port System to remove ground clutter and other spurious echoes. Further editing of aliased velocity values is re- Ϫ1 quired due to the relatively low (12.88 m s ) unam- REFERENCES biguous velocity. In the middle and upper portions of most of the storms, severe speckling (see Wakimoto and Adlerman, E. J., K. K. Droegemeier, and M. Xue, 1996: Numerical Atkins 1996) of the radial velocity precludes velocity simulations of cyclic mesocyclogenesis. Preprints, 18th Conf. on dealiasing. Thus, subsequent data processing is restrict- Severe Local Storms, San Francisco, CA, Amer. Meteor. Soc., 728±732. ed to the lowest several kilometers of the storm. Despite Barnes, S. 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